Eyepiece zoom lens system

ABSTRACT

In an eyepiece zoom lens having a lens configuration constituted by seven lenses in three groups, predetermined conditional expressions are satisfied so as to realize a zoom ratio of 3 while attaining a sufficient eye relief and ameliorating various kinds of aberration. The first lens group G 1  is made stationary, whereas the second lens group G 2  and the third lens group G 3  are made as movable groups which, when changing the power, respectively move in directions opposite to each other on the optical axis while holding therebetween an object image which is formed between these two lens groups. Further, the eyepiece zoom lens is configured so as to satisfy the following conditional expressions: 
     
         5&lt;f.sub.1 /f.sub.m &lt;7, 3.5&lt;f.sub.2 /f.sub.m &lt;4.5,-3.7&lt;f.sub.3 /f.sub.m &lt;-3, 
    
      0.1&lt;S 12  /f m  &lt;1.0, 6.0&lt;S 23  /f m  &lt;8.0,ν 1  &lt;35, ν 2  &lt;35, ν 3  &lt;50

RELATED APPLICATIONS

This application claims the priority of Japanese Patent Application No. 7-260756 filed on Sep. 12, 1995, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an eyepiece zoom lens system used for telescopes, binoculars, and the like.

2 . Description of the Prior Art

In general, in an eyepiece zoom lens system, a movable space for moving the lens system is necessary, whereby the total length of the lens system tends to become long. Accordingly, it is difficult to configure the zoom lens system in a compact size. Though the power of each lens group may be increased in order to attain a compact size, a larger amount of aberration will be generated thereby. Also, the amount of fluctuation in aberration due to zooming increases, thereby making it difficult to favorably correct the aberration while achieving a high zoom ratio. As examples of an eyepiece zoom lens system, those disclosed in Japanese Unexamined Patent Publication No. 53- 51757 and Japanese Patent Publication No. 7- 34061 have been known.

Though the lens systems disclosed in the above-mentioned publications have favorable performances, each of them has h zoom ratio of 2, which is not always sufficient in terms of the high zoom ratio. In recent years, there has been an attempt to increase the zoom ratio of compact cameras from double power zoom toward triple power zoom, while a demand for increasing the zoom ratio has also become higher in telescopes and binoculars.

SUMMARY OF THE INVENTION

In view of such circumstances, the object of the present invention is to provide an eyepiece zoom lens system which has a sufficient eye relief and can ameliorate various kinds of aberration, while attaining a zoom ratio as high as 3.

The eyepiece zoom lens system of the present invention comprises, successively from the eye point side, a first lens group G₁ having a positive refractive power, a second lens group G₂ having a positive refractive power, and a third lens group G₃ having a negative refractive power, wherein the first lens group G₁ is made stationary, whereas the second lens group G₂ and the third lens group G₃ are made as movable groups which, when changing the power, respectively move in directions opposite to each other on the optical axis while holding therebetween an object image which is formed between these two lens groups.

The first lens group G₁ comprises a laminated lens in which a biconcave lens and a biconvex lens which are successively disposed from the eye point side and bonded together; the second lens group G₂ comprises, successively from the eye point side, a laminated lens in which a biconvex lens and a negative meniscus lens whose concave surface faces the eye point are bonded together and a positive single lens; and the third lens group G₃ comprises one or two lenses including a negative lens.

Assuming that the shortest focal length of the whole lens system is f_(m), the focal length of the first lens group G₁ is f₁, the focal length of the second lens group G₂ is f₂, the focal length of the third lens group G₃ is f₃, the principal point distance between the first lens group G₁ and the second lens group G₂ is S₁₂, the principal point distance between the second lens group G₂ and the third lens group G₃ is S₂₃, and the abbe numbers of the negative lenses in the first lens group G₁ , second lens group G₂, and the third lens group G₃ are respectively ν₁, ν₂, and ν₃, the following conditional expressions (1) to (8):

    5<f.sub.1 /f.sub.m <7                                      (1)

    3.5<f.sub.2 /f.sub.m <4.5                                  (2)

    -3.7<f.sub.3 /f.sub.m <-3                                  (3)

    0.1<S.sub.12 /f.sub.m <1.0                                 (4)

    6.0<S.sub.23 /f.sub.m <8.0                                 (5)

    ν.sub.1 <35                                             (6)

    ν.sub.2 <35                                             (7)

    ν.sub.3 >50                                             (8)

are satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a basic lens configuration in accordance with Embodiment 1 of the present invention;

FIG. 2 is a schematic view showing a basic lens configuration in accordance with Embodiment 2 of the present invention;

FIG. 3 is an aberration chart of the lens in accordance with Embodiment 1 (in its shortest focal length state);

FIG. 4 is an aberration chart of the lens in accordance with Embodiment 1 (in its middle state);

FIG. 5 is an aberration chart of the lens in accordance with Embodiment 1 (in its longest focal length state);

FIG. 6 is an aberration chart of the lens in accordance with Embodiment 2 (in its shortest focal length state);

FIG. 7 is an aberration chart of the lens in accordance with Embodiment 2 (in its middle state); and

FIG. 8 is an aberration chart of the lens in accordance with Embodiment 2 (in its longest focal length state).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, eyepiece zoom lens systems in accordance with embodiments of the present invention will be explained with reference to the drawings.

Embodiment 1

FIG. 1 shows the basic lens configuration of Embodiment 1. As shown in FIG. 1, the eyepiece zoom lens system in accordance with this embodiment is disposed within the eyepiece section of a high-power monocular and constituted by seven sheets of lenses L₁, to L₇, namely, comprises, successively from the eye point side, a first lens group G₁, constituted by a biconcave first lens L₁ and a biconvex second lens L₂ ; a second lens group G₂ constituted by a biconvex third lens L₃, a fourth lens L₄ made of a negative meniscus lens whose concave surface faces the eye point, and a fifth lens L₅ whose surface with a larger curvature faces the eye point; and a third lens group G₃ constituted by a biconcave sixth lens L₆ and a positive meniscus lens whose convex surface faces the eye point. Also, the first lens L₁ and second lens L₂, the third lens L₃ and fourth lens L₄, and the sixth lens L₆ and seventh lens L₇ respectively constitute laminated lenses.

Also, the first lens group G₁ is made stationary, whereas the second lens group G₂ and the third lens group G₃ are made as movable groups which, when changing the power, respectively move in directions opposite to each other on the optical axis while holding therebetween an object image which is formed between these two lens groups.

Further, the configuration is made so as to satisfy the following conditions:

    5<f.sub.1 /f.sub.m <7                                      (1)

    3.5<f.sub.2 /f.sub.m <4.5                                  (2)

    -3.7<f.sub.3 /f.sub.m <-3                                  (3)

    0.1<S.sub.12 /f.sub.m <1.0                                 (4)

    6.0<S.sub.23 /f.sub.m <8.0                                 (5)

    ν.sub.1 <35                                             (6)

    ν.sub.2 <35                                             (7)

    ν.sub.3 >50                                             (8)

In the following, the above conditional expressions (1) to (8) will be explained.

Beyond the upper limit of the above conditional expression (1), the refractive power of the first lens group G₁ becomes so weak that the amount of peripheral luminous flux refracted to the optical axis direction becomes small, whereby the power-changing effect of the second lens group G₂ decreases. Accordingly, the load on the third lens group G₃ for changing the power becomes too much, thereby deteriorating the balance in aberration correction and making it difficult to correct aberration. Below the lower limit, on the other hand, the refractive power of the first lens group G₁ becomes so strong that it becomes difficult to correct the aberration of the first lens group G₁.

Beyond the upper limit of the above conditional expression (2), the refractive power of the second lens group G₂ becomes so weak that the amount of movement of the second lens group G₂ for changing the power becomes too much. Below the lower limit, on the other hand, the refractive power of the second lens group G₂ becomes so strong that it becomes difficult to correct the aberration of the second lens group G₂.

Beyond the upper limit of the above conditional expression (3), the refractive power of the third lens group G₃ becomes so strong that it becomes difficult to correct the aberration of the third lens group G₃. In particular, aberration becomes worse when the focal length of the whole system as the eyepiece lens is long. Below the lower limit, on the other hand, the refractive power of the third lens group G₃ becomes so weak that it becomes difficult to correct the distortion in the third lens group G₃.

Beyond the upper limit of the above conditional expression (4), the distance between the first lens group G₁ and the second lens group G₂ becomes so large that the diameter of the second lens group G₂ increases, thereby making it difficult to correct aberration. Below the lower limit, by contract, the distance between the first lens group G₁ and the second lens group. G₂ becomes so small that the first lens group G₁ and the second lens group G₂ interfere with each other, thereby making it difficult to place the lenses.

Beyond the upper link of the above conditional expression (5), the distance between the second lens group G₂ and the third lens group G₃ increases such that the whole length of the lens system becomes too large, whereby a compact size cannot be attained. Below the lower limit, on the other hand, the distance between the second lens group G₂ and the third lens group G₃ becomes so small that the object image formed between the second lens group G₂ and the third lens group G₃ is positioned too close to the second lens group G₂ or the third lens group G₃, whereby dust which has adhered to the lens surface of the second lens group G₂ or third lens group G₃ becomes conspicuous.

The above conditional expression (6) is necessary for favorably correcting chromatic aberration without reducing the radius of curvature of the bonding surface of the positive lens constituting the first lens group G₁.

The above conditional expression (7) is necessary for favorably correcting chromatic aberration without reducing the radius of curvature of the bonding surface of the positive lens constituting the second lens group G₂.

The above conditional expression (8) is necessary for minimizing the fluctuation in chromatic aberration due to changes in the power.

In thus configured eyepiece zoom lens system, an object image I' which has been formed by a non-depicted objective lens is formed by the third lens group G₃ as a slightly enlarged image I between the third lens group G₃ and the second lens group G₂ and then guided to an eye point position E.P. by the second lens group G₂ and the first lens group G₁ so as to form an image again on the retina. Here, the object image formed by the objective lens without the third lens G₃ is indicated by I'.

Also, the image I of the object is moved when the third lens group G₃ is shifted, as indicated by a broken line in FIG. 1, between the shortest focal length state (high-power time as a telescope) and the longest focal length state (low-power time as a telescope). Further, when the second group G₂ is moved as indicated by a broken line in FIG. 1, the luminous flux of the image I of the object is guided onto the eye point E.P., thereby making it form an image on the retina.

Here, FIG. 1 shows the object image I' formed by the objective lens system alone.

Table 1 shows radius of curvature R (mm) of each lens surface, center thickness of each lens and air distance between neighboring lenses d (mm), and refractive index n and abbe number ν of each lens at d line (λ=587.6 nm) in Embodiment 1.

In Table 1 and Table 3, which will be mentioned later, the numbers corresponding to the respective values of each of R, d, n, and ν successively increase from the eye point E.P. side.

                  TABLE 1                                                          ______________________________________                                         R            d            n       ν                                         ______________________________________                                         1       -184.93  1.70         1.80518                                                                              25.4                                       2       31.22    7.11         1.58913                                                                              61.2                                       3       -18.73   d.sub.3 (Variable)                                            4       73.98    5.05         1.58913                                                                              61.2                                       5       -38.00   2.00         1.80518                                                                              25.4                                       6       -48.00   0.50                                                          7       38.56    4.94         1.58913                                                                              61.2                                       8       -263.19  d.sub.s (Variable)                                            9       -59.20   1.50         1.51680                                                                              64.2                                       10      10.42    6.01         1.80518                                                                              25.4                                       11      13.33    B f (Variable)                                                ______________________________________                                    

Also, Table 2 shows the values of d₃ (distance between the first lens group G₁ and the second lens group G₂), d₈ (distance between the second lens group G₂ and the third lens group G₃), and Bf (distance between the apex of the last surface of the lens system in the case where the ray tracing is performed from the eye point side and the virtual image (object image formed by the objective lens system alone) I' formed by the eyepiece lens system; back focus) in cases where f is 7.35 mm, 12.86 mm, and 22.05 mm, respectively.

                  TABLE 2                                                          ______________________________________                                         f        d.sub.3       d.sub.8                                                                               B f                                              ______________________________________                                          7.35     1.05         45.75  -14.24                                           12.86    17.46         25.61  -10.51                                           22.05    28.84          9.05  -5.33                                            ______________________________________                                    

Further, the values in Embodiment 1 are set as follows:

f₁ =45.71, f₂ =28.28, f₃ =-26.41, f₁ /f_(m) =6.22, f₂ /f_(m) =3.85, f₃ /f_(m) =-3.59, S₁₂ /f_(m) =0.36, S₂₃ /f_(m) =7.59, ν₁ =25.4, ν₂ =25.4, and ν₃ =64.2 thereby satisfying all the above conditional expressions (1) to (8).

Also, in the lens system of Embodiment 1, the values of the whole focal length f and field angle 2ω are as follows:

focal length f=7.35 mm to 22.05 mm

field angle 2ω=64° to 40° whereby the zoom ratio is 3.

Embodiment 2

FIG. 2 shows the lens configuration of Embodiment 2. Its lens configuration and conditional expressions (1) to (8) are substantially the same as those of Embodiment 1 except that the fifth lens L₅ is a positive meniscus lens whose convex surface faces the eye point.

Table 3 shows radius of curvature R (mm) of each lens surface, center thickness of each lens and air distance between neighboring lenses d (mm), and refractive index n and abbe number ν of each lens at d line in Embodiment 2.

                  TABLE 3                                                          ______________________________________                                         R            d            n       ν                                         ______________________________________                                         1       -233.21  1.70         1.80518                                                                              25.4                                       2       31.51    7.39         1.58913                                                                              61.2                                       3       -19.18   d.sub.3 (Variable)                                            4       52.28    5.88         1.58913                                                                              61.2                                       5       -36.27   2.00         1.80518                                                                              25.4                                       6       -40.14   0.50                                                          7       44.94    3.70         1.58913                                                                              61.2                                       8       200.00   d.sub.8 (Variable)                                            9       -79.62   1.50         1.51680                                                                              64.2                                       10      10.07    5.80         1.80518                                                                              25.4                                       11      12.36    B f (Variable)                                                ______________________________________                                    

Also, Table 4 shows the values of d₃ (distance between the first lens group G₁ and the second lens group G₂), d₈ (distance between the second lens group G₂ and the third lens group G₃), and Bf (distance between the apex of the last surface of the lens system in the case where the ray tracing is performed from the eye point side and the virtual image (object image formed by the objective lens system alone) I' formed by the eyepiece lens system; back focus) in cases where f is 7.34 mm, 12.85 mm, and 22.02 mm, respectively.

                  TABLE 4                                                          ______________________________________                                         f        d.sub.3       d.sub.8                                                                               B f                                              ______________________________________                                          7.34     1.08         45.61  -13.84                                           12.85    17.63         25.43  -10.20                                           22.02    29.21          8.83   -5.19                                           ______________________________________                                    

Further, the values in Embodiment 2 are set as follows:

f₁ =45.55, f₂ =29.02, f₃ =-25.94, f₁ /f_(m) =6.21, f₂ /f_(m) =3.95, f₃ /f_(m) =-3.53, S₁₂ /f_(m) =0.31, S₂₃ /f_(m) =7.64, ν₁ =25.4, ν₂ =25.4,and ν₃ =64.2 thereby satisfying all the above conditional expressions (1) to (8).

Also, in the lens system of Embodiment 2, the values of the whole focal length f and field angle 2ω are as follows:

focal length f=7.34 mm to 22.02 mm

field angle 2ω=64° to 40° whereby the zoom ratio is 3.

FIGS. 3, 4, and 5 respectively show aberration charts (each showing spherical aberration, astigmatism, distortion, and coma) in Embodiment 1 in its shortest focal length state (high-power time as a telescope), intermediate state, and longest focal length state (low-power time as a telescope); whereas FIGS. 6, 7, and 8 respectively show aberration charts (each showing spherical aberration, astigmatism, distortion, and coma) in Embodiment 2 in its shortest focal length state (high-power time as a telescope), intermediate state, and longest focal length state (low-power time as a telescope).

As can be seen from FIGS. 3 to 8, in the above-mentioned embodiments, various kinds of aberration can be ameliorated, whereby performances which can be favorably used as an eyepiece zoom lens system can be attained.

Without being restricted to the above-mentioned embodiments, the zoom lens system of the present invention can be modified in various manners. For example, the radius of curvature r and the distance (or lens thickness) d can be appropriately modified.

As explained in the foregoing, in accordance with the eyepiece zoom lens system of the present invention, since it has a lens configuration comprising seven lenses in three groups and satisfies predetermined conditional expressions, a zoom ratio of 3 can be attained while having a sufficient eye relief and favorably correcting various kinds of aberration. 

What is claimed is:
 1. An eyepiece zoom lens system comprising, successively from an eye point side, a first lens group G₁ having a positive refractive power, a second lens group G₂ having a positive refractive power, and a third lens group G₃ having a negative refractive power, said first lens group G₁ being stationary, whereas said second lens group G₂ and said third lens group G₃ being movable groups which, when changing power, respectively move in directions opposite to each other on an optical axis while holding therebetween an object image which is formed between these two lens groups,wherein said first lens group G₁ comprises a laminated lens in which a biconcave lens and a biconvex lens which are successively disposed from the eye point side and bonded together; said second lens group G₂ comprises, successively from the eye point side, a laminated lens in which a biconvex lens and a negative meniscus lens whose concave surface faces the eye point are bonded together and a positive single lens; and said third lens group G₃ comprises one or two lenses including a negative lens and wherein, assuming that shortest focal length of the whole lens system is f_(m), focal length of said first lens group G₁ is f₁, focal length of said second lens group G₂ is f₂, focal length of said third lens group G₃ is f₃, principal point distance between said first lens group G₁ and said second lens group G₂ is S₁₂, principal point distance between said second lens group G₂ and said third lens group G₃ is S₂₃, and abbe numbers of the negative lenses in said first lens group G₁, second lens group G₂, and third lens group G₃ are respectively ν₁, ν₂, and ν₃, the following conditional expressions (1) to (8):

    5<f.sub.1 /f.sub.m <7                                      (1)

    3.5<f.sub.2 /f.sub.m <4.5                                  (2)

    -3.7<f.sub.3 /f.sub.m <-3                                  (3)

    0.1<S.sub.12 /f.sub.m <1.0                                 (4)

    6.0<S.sub.23 /f.sub.m <8.0                                 (5)

    ν.sub.1 <35                                             (6)

    ν.sub.2 <35                                             (7)

    ν.sub.3 >50                                             (8)

are satisfied. 